People with spinal cord injury (SCI) may have some walking ability, however, they are often challenged with a spastic gait pattern, characterized by poor voluntary control of the muscles that lift the foot (dorsiflexors) and spasms of the muscles that control push-off (plantar flexors). Both deficits contribute to impaired ability to achieve foot-ground clearance, making walking laborious and unstable. Two major neural contributors to spastic gait include disrupted descending corticospinal tract (CST) drive to the ankle dorsiflexors and impaired modulation of the plantar flexor spinal reflex circuit (SRC). It is still unknown whether diminished descending volitional drive or impairment of reflex modulation contributes more to spastic gait. Afferent input has a strong neuromodulatory effect on the excitability of both the CST and SRC. Whole body vibration (WBV) is a robust form of afferent input that provides a probe to understand the neurophysiologic mechanisms underlying spastic gait. The long-term goal of this research is to develop rehabilitation strategies to enhance walking function and mobility in persons with SCI by improving ankle control. The overall objective of this proposal is to understand the roles of the CST and SRC in volitional and reflex control of the ankle. The rationale for this project is that understanding the neurophysiological contributions to increased volitional control and decreased spasticity in the ankles will be the first step to developing important non-invasive rehabilitation strategies to improve spastic gait in persons with SCI and other upper motor neuron disorders. The central hypothesis is that improvements in ankle function will have a greater association with changes in the CST than with changes in SRC modulation, as corticospinal drive influences both volitional control and reflex modulation. Using WBV to activate cortical and spinal circuits, the central hypothesis will be tested via two specific aims: 1) Determine the contribution of dorsiflexors CST and plantar flexor SRC excitability to ankle volitional control in individuals with SCI; and 2) Determine the contribution of dorsiflexors CST and plantar flexor SRC excitability to ankle spasticity in individuals with SCI. The proposed research is innovative, because there has yet to be a study examining how an intervention that targets the CST and SRC simultaneously, impacts the different components of spastic gait after SCI. Understanding the mechanisms underlying spastic gait is significant because it provides a foundation for the development of targeted non-invasive therapies to increase the functional outcomes of the people impacted by this impaired gait pattern. Overall, the knowledge gained in this proposal will provide more insight in the underlying mechanisms of ankle control and spasticity in persons with SCI.